With that being said, there would have to be some method for developers to capture the screen of their iOS 8 devices – right?

As 9to5Mac reports, OS X 10.10 Yosemite actually comes with a feature that will allow iOS 8 device users to plug their device into their Mac via USB and the operating system will recognize the iOS 8 device as a type of camera that can be recorded from, as shown above.

The recording supports both video and sound, along with the ability to choose a video recording quality, and this makes the perfect combination for an application preview in the App Store.

Apple's developer page has the information about capturing your screen on OS X Yosemite, and it works for iPhone, iPod touch, or iPad:

How Do I Create an App Preview?

With iOS 8 and OS X Yosemite, you can capture real-time footage of your app directly off your iPhone, iPad, or iPod touch. Just connect your device to your Mac using the Lightning connector and it will be automatically available as a video camera. You can capture anything you’re doing on-screen directly to your Mac using QuickTime Player. Edit your captured footage in your favorite video editing app and upload it iTunes Connect—just like your screenshots—to submit it for review along with your next app update.

It would seem that the feature will not support the iPhone 4s, as it only supports devices that use the Lightning Connector. The developer page says that only devices with a four-inch retina display or iPads are supported.

The display capture is done through QuickTime and can be edited later on before being submitted to the App Store. Obviously, there are several other reasons too for why capturing your display could be helpful, and using this method of display capture would not be limited to Apple's application preview use only.